Manufactured silk could
be used for artificial tendons and ligaments, sutures, parachutes
and bulletproof vests

Deborah HalberOctober 30,
2006

Golden-Silk Spider on WebMIT
researchers hope the golden silk spider will help them
figure out how to produce strong, durable silk artificially.

A team of MIT engineers has
identified two key physical processes that lend spider silk its
unrivaled strength and durability, bringing closer to reality the
long-sought goal of spinning artificial spider silk.

Manufactured spider silk could
be used for artificial tendons and ligaments, sutures, parachutes
and bulletproof vests. But engineers have not managed to do what
spiders do effortlessly.

In a study published in the
November issue of the Journal of Experimental Biology, Gareth H.
McKinley, professor of mechanical engineering, and colleagues
examined how spiders spin their native silk fibers, with hopes of
ultimately reproducing the process artificially.

McKinley heads the
Non-Newtonian Fluid Dynamics research group in MIT's Department
of Mechanical Engineering. Non-Newtonian fluids behave in strange
and unexpected ways because their viscosity, or consistency,
changes with both the rate and the total amount of strain applied
to them.

Spider silk is a protein
solution that undergoes pronounced changes as part of the
spinning process. Egg whites, another non-Newtonian fluid, change
from a watery gel to a rubbery solid when heated. Spider silk, it
turns out, undergoes similar irreversible physical changes.

Stickiness
and flow

McKinley and Nikola Kojic, a
graduate student in the Harvard-MIT Division of Health Sciences
and Technology, studied the silk of Nephila clavipes, the golden
silk orb-weaving spider. One species of golden orb spider creates
a web so strong it can catch small birds. In the South Pacific,
people make fishing nets out of this web silk.

The researchers chose the
golden silk spider because of the formidable strength of its web.
But Kojic was taken aback when the first palm-sized spider
crawled out of the box he received in the mail from an
accommodating employee of Miami's MetroZoo. (She simply gathered
some up from the grounds; the zoo does not exhibit golden orb
spiders.)

"This is pretty scary,"
he said. "I'd never seen a spider this big. I never grew up
around anything with furry knuckles." But he quickly settled
into dissecting the peanut-sized and -shaped protuberance on the
spiders' backs containing their silk-producing glands and
spinnerets.

Spiders don't actually spin
("spinning" refers to the age-old art of drawing out
and twisting fibers to form thread); instead, they squirt out a
thick gel of silk solution. (One teaspoonful can make 10,000
webs.) They then use their hind legs as well as their body weight
and gravity to elongate the gel into a fine thread.

Kojic, who first practiced on
silkworms, learned how to extract a microscopic amount of the
gel-like solution from the spider's silk-producing major
ampullate gland.

The researchers used devices
called micro-rheometers--custom-made to handle the tiny drops of
silk solution--to test the material's behavior when subjected to
forces. The team tested the thick solution's viscosity, or how it
flowed, by "shearing" it, or placing it between two
rapidly moving glass plates. They tested its stickiness by
pulling it apart, like taffy, between two metal plates.

The magic that makes silk so
strong, the researchers discovered, happens while it flows out of
the spider's gland, lengthens into a filament and dries.

Engineering
nature

The key to spider silk is
polymers.

Plastics, Kevlar (used in
bulletproof vests) and parts of the International Space Station
are some of the many items made from polymers. The proteins in
our bodies are polymers made from amino acids. From the Greek for
"many" and "units," polymers are long linked
chains of small molecules. They can be flexible or stiff,
water-soluble or insoluble, resistant to heat and chemicals and
very strong.

Silk protein solution consists
of 30-40 percent polymers; the rest is water. The spider's
silk-producing glands are capable of synthesizing large fibrous
proteins and processing those proteins into an insoluble fiber.

"The amazing thing nature
has found is how to spin a material out of an aqueous solution
and produce a fiber that doesn't re-dissolve," McKinley
said. Like a cooked egg white, dry spider silk doesn't revert to
its former liquid state. What started out as a water-based
solution becomes impervious to water.

The silk protein's long
molecules are like tangled spaghetti. They form a viscous
solution but are slippery enough to slide past each other easily
and squeeze through the spider's ampullate gland. As the silk gel
flows from the gland through an S-shaped, tapered canal to the
outside of the spider's body, the long protein molecules become
aligned and the viscosity (or resistance to flow) drops by a
factor of 500 or more.

As the resulting liquid exits
the abdomen through the spinneret, it has the characteristics of
a liquid crystal. It's the exquisite alignment of the protein
fibers, Kojic said, that gives silk threads their amazing
strength.

While the silk stretches and
dries, it forms miniscule crystalline structures that act as
reinforcing agents. Engineered nanoparticles--tiny materials
suspended in artificial silk--may be able to serve the same
purpose.

In conjunction with the polymer
synthesis and analysis work of Paula T. Hammond, an MIT professor
of chemical engineering, McKinley's laboratory will use the new
insights about spider silk to team up with MIT's Institute for
Soldier Nanotechnologies to emulate the properties of silk
through polymer processing.

"We're interested in
artificial materials that emulate silk," McKinley said.
Tailoring the properties of the liquid artificial spinning
material to match the properties of the real thing "may
prove essential in enabling us to successfully process novel
synthetic materials with mechanical properties comparable to, or
better than, those of natural spider silk," the authors
wrote.

This work was supported by the
NASA Biologically Inspired Technology Program, the DuPont-MIT
Alliance and the MIT Institute for Soldier Nanotechnologies.